Bi-directional battery voltage converter

ABSTRACT

A method of charging batteries in a vehicle electrical system having a system bus, a first switch selectively connecting a first battery with the system bus, a second switch selectively connecting a second battery with the system bus, and a controller monitoring a voltage of the vehicle electrical system and controllably opening and closing the first switch and the second switch. The method includes closing the first switch and opening the first switch if the voltage of the vehicle electrical system traverses a threshold value. The method also includes closing the second switch and opening the second switch if the voltage of the vehicle electrical system traverses the threshold value.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 61/230,296 filed on Jul. 31, 2009, and U.S. Provisional PatentApplication No. 61/315,759 filed on Mar. 19, 2010, each of which isincorporated herein by reference in its entirety.

BACKGROUND

The invention relates to a control system for electrical storageelements of a vehicle, and in one particular embodiment, to a system forcontrolling power into and out of the electrical storage elements.

Large vehicles (e.g., semi-tractors, trucks, buses, etc.) are commonlyused to transport people and cargo. The vehicles include variouscomponents that draw electrical power, including for example a heating,ventilation, and air conditioning (HVAC) system for a sleeper unit in along-distance tractor. The power for these various electrical componentsmay be supplied by an alternator while the vehicle is in operation andby an alternative power source, such as one or more batteries, when thevehicle is not in operation.

In general, electrical energy from a power source, such as thealternator, is stored in one or more batteries of the vehicle to providestored electrical energy for later use when other power sources areunavailable. In some vehicles, groups of auxiliary batteries areprovided for supplying power to electrical components of the vehicle.These groups of auxiliary batteries are often electrically connected tothe power source in a parallel relationship to one another.

SUMMARY

In one embodiment, the invention includes a bi-directional batteryvoltage converter for a vehicle electrical system. The bi-directionalbattery voltage converter includes at least one battery. Each batteryhas associated therewith a power source, an inductor, four switcheselectrically coupled to the inductor where two of the switches are alsoelectrically coupled to the battery and the other two of the switchesare electrically coupled to the power source, and a routing circuitconnected to each of the switches. The routing circuit controls theopening and closing of each of the switches such that the switches areopened or closed in pairs. By alternately coupling the inductor to thepower source and then the battery in a defined duty cycle, the routingcircuit charges the inductor using the power source and then transfersthe charge stored in the inductor to the battery. In variousembodiments, the power source is a vehicle alternator.

In another embodiment, the invention includes a method of individuallycharging each of a plurality of batteries in a vehicle electricalsystem. The method includes providing a bi-directional battery voltageconverter electrically coupled to each individual battery. Thebi-directional battery voltage converter can include a current sensorassociated with the inductor and a voltage sensor associated with thebattery. The method further includes sensing the voltage of each batteryand the current flowing into each battery and adjusting the duty cycleof the bi-directional battery voltage converter to adjust at least oneof the current and the voltage being delivered to the battery, so as toprovide optimal charging to each individual battery.

In yet another embodiment, the invention includes a method of managingrecharging of a plurality of batteries in a vehicle electrical system.The method includes providing an electronic switch electrically coupledto each of a plurality of batteries in a vehicle electrical system and avoltage sensor to sense the voltage of a power source connected to thevehicle electrical system. The method further includes closing a firstswitch to connect a first of the plurality of batteries to the vehicleelectrical system and monitoring the voltage of the power source whenthe first switch is closed. The method also includes determining whetherthe voltage has decreased below a threshold level due to the firstbattery being connected and, if so, reopening the first switch todisconnect the first battery. The method still further includes closingeach of the plurality of switches associated with each of the pluralityof batteries and monitoring the voltage until all of the switches areclosed and all of the batteries are connected to the vehicle electricalsystem for recharging.

In still another embodiment, the invention includes a method ofindividually discharging each of a plurality of batteries in a vehicleelectrical system. The method includes providing a bi-directionalbattery voltage converter electrically coupled to each individualbattery, where the bi-directional battery voltage converter alsoincludes a current sensor associated with the inductor and a voltagesensor associated with the battery. The method further includes sensingthe voltage of each battery and the current provided by each battery,and adjusting the duty cycle of the bi-directional battery voltageconverter to adjust at least one of the current and the voltage beingdelivered by the battery, so as to provide optimal discharging of eachindividual battery.

In yet another embodiment, the invention includes a method ofelectrically isolating at least one battery in a vehicle electricalsystem. The method includes providing an electronic switch electricallycoupled to each of a plurality of batteries in a vehicle electricalsystem. The method further includes, during discharging of the pluralityof batteries, opening at least one switch to electrically isolate atleast one of the plurality of batteries to preserve charge within theisolated battery.

In still another embodiment, the invention includes a bi-directionalmulti-battery voltage converter for a vehicle electrical system. Eachbi-directional multi-battery voltage converter is associated with aplurality of batteries. A control circuit is provided to selectivelyenergize a relay or electronic switch associated with one of theplurality of batteries, thereby connecting the battery with thebi-directional multi-battery voltage converter.

In still yet another embodiment, the invention provides a method ofcharging batteries in a vehicle electrical system. The vehicleelectrical system has a system bus, a first switch selectivelyconnecting a first battery with the system bus, a second switchselectively connecting a second battery with the system bus, and acontroller monitoring a voltage of the vehicle electrical system andcontrollably opening and closing the first switch and the second switch.The first switch and the second switch are opened, thereby disconnectingthe first battery and the second battery from the system bus. The firstswitch is closed, thereby connecting the first battery with the systembus. The voltage of the vehicle electrical system is monitored inresponse to the first switch closing. The first switch is opened if thevoltage of the vehicle electrical system traverses a threshold value.The second switch is closed, thereby connecting the second battery withthe system bus. The voltage of the vehicle electrical system ismonitored in response to the second switch closing. The second switch isopened if the voltage of the vehicle electrical system traverses thethreshold value.

In still yet another embodiment, the invention provides a controller foruse with a vehicle electrical system. The controller has a voltage inputterminal, a memory, and a micro-processor. The controller controllablyopens a first switch, thereby disconnecting a first battery from asystem bus and controllably opens a second switch, thereby disconnectinga second battery from the system bus. The controller controllably closesthe first switch, thereby connecting the first battery with the systembus. The controller receives a voltage of the vehicle electrical systemat the voltage input terminal in response to the first switch closing.The controller then compares, in the micro-processor, a value related tothe voltage of the vehicle electrical system to a threshold value fromthe memory. The controller controllably opens the first switch inresponse to the voltage of the vehicle electrical system traversing athreshold value. The controller controllably closes the second switch,thereby connecting a second battery with the system bus. The controllerreceives a voltage of the vehicle electrical system at the voltage inputterminal in response to the second switch closing. The controller thencompares, in the micro-processor, a value related to the voltage of thevehicle electrical system to the threshold value, and controllably opensthe second switch in response to the voltage of the vehicle electricalsystem traversing the threshold value.

In still yet another embodiment, the invention provides a vehicleelectrical system including a system bus, a first battery, a firstswitch selectively connecting the first battery to the system bus, asecond battery, a second switch selectively connecting the secondbattery to the electrical system, and a controller. The controllercontrollably opens the first switch and the second switch, therebydisconnecting the first battery and the second battery from the systembus. The controller controllably closes the first switch, therebyconnecting the first battery with the system bus, and monitors a voltageof the vehicle electrical system in response to the first switchclosing. The controller controllably opens the first switch if thevoltage of the vehicle electrical system traverses a threshold value.The controller controllably closes the second switch, thereby connectinga second battery associated with the second switch with the system bus,and monitors the voltage of the vehicle electrical system in response tothe second switch closing. The controller opens the second switch if thevoltage of the vehicle electrical system traverses the threshold value.

In still yet another embodiment, the invention provides a method ofbalancing current in a vehicle electric system. The vehicle electricalsystem includes a system bus, a first battery, a first bi-directionalbattery voltage converter selectively transferring a first currentbetween the first battery and the system bus, a second battery, a secondbi-directional battery voltage converter selectively transferring asecond current between the second battery and the system bus, and acontroller controlling the first bi-directional battery voltageconverter and the second bi-directional battery voltage converter. Thefirst current is sensed and a first signal related to the first currentis provided to the controller. The second current and a second signalrelated to the second current is provided to the controller. The firstbi-directional battery voltage converter and the second bi-directionalbattery voltage converter are controlled so that the first current andthe second current are equal portions of a load current supplied to anelectrical load connected to the system bus.

In still yet another embodiment, the invention provides a vehicleelectrical system for supplying electrical power to an electrical load.The system includes a system bus, a first battery, and a firstbi-directional battery voltage converter controllably transferring afirst current between the first battery and the system bus. The systemalso includes a second battery, and a second bi-directional batteryvoltage converter controllably transferring a second current between thesecond battery and the system bus. A controller controls the firstbi-directional battery voltage converter and the second bi-directionalbattery voltage converter such that the first current and the secondcurrent are equal portions of a load current supplied to an electricalload connected to the system bus.

In still yet another embodiment, the invention provides a bi-directionalbattery voltage converter for use with a vehicle electrical system. Thebi-directional battery voltage converter includes an inductor, a firstswitch selectively coupling the inductor to a first battery, a secondswitch selectively coupling the inductor to the first battery, a thirdswitch selectively coupling the inductor to the vehicle electricalsystem, and a fourth switch selectively coupling the inductor to thevehicle electrical system. A routing circuit is connected to each of thefirst, second, third, and fourth switches. The routing circuitcontrollably opens and closes the switches in pairs such that theinductor is charged from one of the vehicle electrical system and thebattery and discharged to the other of the vehicle electrical system andthe battery. A controller controls the routing circuit to deliver aportion of a load current supplied to a connected electrical load, theportion based upon the availability of other current sources.

In still yet another embodiment, the invention provides an electricalsystem for a vehicle. The electrical system includes a system bus, anignition switch selecting an operational state of the electrical system,and a primary battery connected to the system bus. A first auxiliarybattery module is connected to the system bus. The first auxiliarybattery module includes a first auxiliary battery, a second auxiliarybattery, a bi-directional battery voltage converter, and a modulecontroller selectively connecting one of the first auxiliary battery andthe second auxiliary battery to the bi-directional battery voltageconverter. A main system controller operates the first auxiliary batterymodule in one of a null mode, wherein the first auxiliary battery andthe second auxiliary battery are disconnected from the electricalsystem, a charging mode, wherein one of the first auxiliary battery andthe second auxiliary battery receives a current via the bi-directionalbattery voltage converter, and a discharging mode, wherein one of thefirst auxiliary battery and the second auxiliary battery supplies acurrent via the bi-directional battery voltage converter.

In still yet another embodiment, the invention provides a battery modulefor use with a vehicle electrical system. The battery module includes abi-directional battery voltage converter, a first battery, and a secondbattery. A first relay selectively connects the first battery to thebi-directional battery voltage converter. A second relay selectivelyconnects the second battery to the bi-directional battery voltageconverter. A controller selectively energizes the first relay,selectively energizes the second relay, and controls a direction ofcurrent through the bi-directional battery voltage converter.

In still yet another embodiment, the invention provides an electricalsystem for a vehicle. The electrical system includes a system bus, anignition switch selecting an operational state of the electrical system,a primary battery connected to the system bus, a first auxiliary batterymodule and a second auxiliary battery module. The first auxiliarybattery module includes a first auxiliary battery, a second auxiliarybattery, a first bi-directional battery voltage converter, and a firstmodule controller selectively connecting one of the first auxiliarybattery and the second auxiliary battery to the bi-directional batteryvoltage converter. The second auxiliary battery module includes a thirdauxiliary battery, a fourth auxiliary battery, a second bi-directionalbattery voltage converter, and a second module controller selectivelyconnecting one of the first auxiliary battery and the second auxiliarybattery to the bi-directional battery voltage converter. A main systemcontroller operates the first auxiliary battery module and the secondauxiliary battery module to prioritize a recharging of one of the first,second, third, and fourth auxiliary batteries.

Various aspects of the invention will become apparent by considerationof the detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a vehicle including a control system, aplurality of electrical storage elements, a power source, and a heating,ventilation, and air conditioning (HVAC) system.

FIG. 2 is a diagram of a portion of a vehicle electrical system, whichincludes a bi-directional battery voltage converter circuit.

FIG. 3 is a diagram of a portion of a vehicle electrical system, whichincludes a bi-directional battery voltage converter circuit.

FIG. 4 is a diagram of a portion of a vehicle electrical system, whichincludes a main vehicle system battery and a plurality of auxiliarybatteries, where the auxiliary batteries have a bi-directional batteryvoltage converter connected in series.

FIG. 5 is a diagram of a portion of a vehicle electrical system, whichincludes a main vehicle system battery and a plurality of auxiliarybatteries, where the auxiliary batteries have an electronic switchconnected in series.

FIG. 6 is a diagram of a portion of a vehicle electrical system, whichincludes a bi-directional multi-battery voltage converter circuit.

FIG. 7 is a diagram of a portion of a vehicle electrical system, whichincludes a bi-directional multi-battery voltage converter circuit.

FIG. 8 is a diagram of a portion of a vehicle electrical system, whichincludes a main vehicle system battery and a plurality of auxiliarybatteries, where the auxiliary batteries have a bi-directionalmulti-battery voltage converter connected in series.

FIG. 9 is a diagram of a bi-direction multi-battery voltage convertercircuit module.

FIG. 10 is a diagram of a vehicle electrical system including thebi-directional multi-battery voltage converter circuit module of FIG. 9.

FIG. 11 is a graph of battery charging voltage and current according toone aspect of the vehicle electrical system of FIG. 10.

FIG. 12 is a control flowchart for one aspect of the vehicle electricalsystem of FIG. 10 while in a charging mode.

FIG. 13 is a control flowchart for one aspect of the vehicle electricalsystem of FIG. 10 while in a null mode.

FIG. 14 is a control flowchart for one aspect of the vehicle electricalsystem of FIG. 10 while in a discharging mode.

DETAILED DESCRIPTION

Before any constructions of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other constructions and of being practicedor of being carried out in various ways. Also, it is to be understoodthat the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having” and variations thereof herein ismeant to encompass the items listed thereafter and equivalents thereofas well as additional items. Further, unless specified or otherwiselimited, “connected” and “coupled” are not restricted to physical ormechanical connections or couplings.

To supply the power needs of a vehicle, including a large vehicle suchas a straight truck or a semi-tractor for pulling a trailer, multiplebatteries are often coupled together to deliver greater power. In onetypical arrangement for coupling multiple batteries, two or morebatteries are wired together in parallel until the total power-deliverycapacity (e.g. measured in peak amperes at a given voltage, such as 12volts) is sufficient for supplying the power needs of the vehicle. Inanother common arrangement, a simple electronic component, such as abattery relay, may be used to electrically couple several batteriestogether. However, neither of these arrangements provides for individualmonitoring and control of each of the batteries, and thus, thearrangements are indifferent to the charging and discharging limitationsof individual batteries. Also, the engagement of two or more batteriescan create large momentary currents, which are inefficient and which mayproduce wear on electronic components in the system.

Known battery charging control systems typically charge an entire groupof parallel-connected batteries by connecting a single power source(e.g. the alternator of the vehicle) to the entire group of batteries.When these batteries have relatively large charge acceptance capacities(i.e., low internal resistance), the current acceptance of the batteriescan exceed the current supplied from the power source. In addition,conventional control systems control the supply voltage to protectagainst over-current charging conditions, which can be detrimental tobattery life. Often, such systems do not provide an appropriate initialamount of current to the batteries, limiting the effective life of thebatteries.

By simultaneously charging entire groups of batteries, conventionalbattery charging control systems typically require that each of thebatteries in the group have the same electrical characteristics,including for example internal resistance, tolerances, and architecture.When the electric current demand from each of the batteries exceeds thecurrent capacity of the power source that is charging the batteries, oneor both of the power source and the batteries may be damaged or operateinefficiently. Excessive electric current demand from the group ofbatteries may also provide inadequate charging of the batteries,lowering at least one of the electrical storage capacity of thebatteries and the cycling capability of the batteries.

Thus, various embodiments of the invention provide apparatus, systems,and methods for managing the charging and discharging of individualbatteries within a group of batteries. Various other embodiments of theinvention provide apparatus, systems, and methods for managing thecharging and discharging of sub-groups of batteries with a group ofbatteries.

FIG. 1 shows an exemplary large vehicle embodying the invention, namelya vehicle 10 for over-the-road operation. The illustrated vehicle 10 isa semi-tractor that can be used to transport cargo stored in a cargocompartment (e.g., a container, a trailer, etc.) to one or moredestinations. It is also envisioned that the invention may also beembodied in other vehicles such as a straight truck, van, bus, camper,car, motorcycle, boat, train, and aircraft, among other possibilities.In still other constructions, the invention could be implemented in abuilding or other setting where multiple batteries are employed.

The vehicle 10 includes a frame 15, wheels 20, a prime mover 25, a fuelreservoir 30, and a direct current (“DC”) generator or alternator 35.The wheels 20 are rotatably coupled to the frame 15 to permit movementof the vehicle 10. The alternator 35 is coupled to the prime mover 25 sothat mechanical energy produced by the prime mover 25 can be convertedinto electrical energy, i.e. electricity. The prime mover 25 may be anengine that runs on diesel fuel, gasoline, or other suitable material.The alternator 35 and the prime mover 25 cooperate to define a firstelectrical power source 40 for the vehicle 10. The first power source 40has a first power capacity that is based on the available electricalpower from the alternator 35 at a predetermined voltage (e.g. 12 volts).

In certain constructions, the prime mover 25 may be an electric motorthat is powered by an internal or external power supply, for example bywires or rails as in a train system or by storing grid power as in abattery-powered, plug-in electric vehicle. In the latter case of acompletely plug-in electric vehicle (i.e. one that does not include anon-board engine to supplement battery power), there may be a particularset of batteries dedicated to storing charge for powering the primemover 25 during operation of the vehicle 10. The batteries for poweringthe prime mover 25 during operation of the vehicle may be separate froma set of auxiliary batteries used to power accessories (e.g. lights andHVAC) when the vehicle 10 is not in operation. In other constructions inwhich the prime mover 25 is an electric motor, the electric motor may bepowered by an on-board power source such as a generator, where thegenerator runs on diesel fuel, gasoline, or other suitable material, asin a hybrid electric vehicle. In the case where the prime mover 25 is anelectric motor, power for charging the various batteries may come fromthe same source that is powering the electric motor, instead of thealternator. In some designs (e.g. an electric vehicle that operates onlyon grid or battery power), the vehicle may not include an alternator.

The prime mover 25 is coupled to the frame 15 and is disposed in acompartment 45 adjacent a forward end of the vehicle 10. The prime mover25 is in communication with one or more of the wheels 20 to drive thevehicle 10. The prime mover 25 can be in an “On” state and an “Off”state. When the prime mover 25 is in the “On” state, it may be engagedwith the wheels 20. In addition, when the prime mover 25 is in the “On”state, it can provide power to the electrical system of the vehicle 10to power loads and to charge batteries in the system. When the primemover 25 is “On” and is engaged with the wheels 20, the vehicle 10 canbe driven. If the prime mover 25 is “On” but is not engaged with thewheels 20, the prime mover 25 and the vehicle 10 are said to be idling,although the prime mover 25 in the idling state can still provide powerto the electrical system of the vehicle 10.

When the vehicle 10 is not going to be operated for a prolonged periodof time (e.g. during an overnight stop or during loading or unloading ofcargo), the prime mover 25 may be put into an “Off” state and thevehicle 10 put into standby mode. In the “Off” state, the prime mover 25is not available to provide power to the electrical system of thevehicle 10. Thus, one or more batteries may be needed to powerelectrical loads in the vehicle 10.

Referring to FIG. 1, the vehicle 10 also includes a cabin 50 and anelectrically powered heating, ventilation, and air conditioning (HVAC)system 55. The HVAC system 55 defines an exemplary electrical load ofthe vehicle 10. The vehicle 10 also may include other electrical loads(e.g., vehicle accessories, lights, starter motor for prime mover 25,etc.). Generally, the electrical load has power characteristics thatrelate to a load draw, which corresponds to the electrical power that isnecessary for adequately powering the load. In addition, charging ofbatteries on the vehicle 10 constitutes another type of load.

The cabin 50 is supported on the frame 15 rearward of the compartment 45and includes walls 60 that define a space 65. In some constructions, thespace 65 may be divided into a driving portion and a sleeping portion.The HVAC system 55 is coupled to the vehicle 10 and is in communicationwith the cabin 50 to condition the space 65. The illustrated vehicle 10includes a single HVAC system 55 that is located adjacent and incommunication with the space 65. In other constructions, the HVAC system55 can be positioned in the vehicle to condition the sleeping portion,and another HVAC system can be positioned in the vehicle to conditionthe driving portion. Generally, the number of HVAC systems in thevehicle depends at least in part on the size and number of zones to beconditioned within the cabin.

Components of the HVAC system 55 can be located almost anywhere on thevehicle 10. In the illustrated construction, the HVAC system 55 includesan evaporator assembly 70 that is located in the cabin 50 to conditionthe space 65, and a condenser assembly 75 that is coupled to one of thewalls 60 on an exterior side of the cabin 50 to provide heat exchangebetween refrigerant in the HVAC system 55 and an ambient environment. Insome constructions, the components of the HVAC system 55 can beassembled together into a single, unitary package. In otherconstructions, each component of the HVAC system 55 can be separate fromthe other components of the HVAC system 55.

FIG. 1 shows that the vehicle 10 also includes an electrical storagesystem 80 and a charge control system 85 in communication with theelectrical storage system 80. The electrical storage system 80 is inelectrical communication with the first power source 40 for receivingelectrical power when the prime mover 25 is in the “On” state. Thecharge control system 85 also may be in selective electricalcommunication with a second electrical power source 90 in addition to,or in lieu of, the first power source 40 for receiving electrical powerfrom the second power source 90. In the illustrated construction, thesecond power source 90 can include power from a municipal grid (alsocalled “shore power”), a photovoltaic device, a fuel cell, a windgenerator, or other sources of power. Generally, the second power source90 has a second electrical power capacity that is based on the availableelectrical power from the power source at a preferred voltage.

The electrical storage system 80 also is in electrical communicationwith the electrical load of the vehicle (e.g., the HVAC system 55) toprovide adequate power to the electrical load based on the load draw.Generally, the electrical storage system 80 receives power from eitheror both of the first power source 40 and the second power source 90during a charge phase, and discharges power to the load (or loads) ofthe vehicle 10 during a discharge phase. A charge phase may occur wheneither or both of the first and second power sources 40, 90 areinputting power to the electrical system of the vehicle 10, and adischarge phase may occur when neither of the first or second powersources 40, 90 are inputting power to the electrical system of thevehicle 10.

The electrical storage system 80 includes a first plurality ofelectrical storage elements (e.g. batteries 95) and a second pluralityof electrical storage elements (e.g. batteries 100) for storingelectrical power from the first power source 40 and/or from the secondpower source 90 during the charge phase, and for discharging power tothe electrical load during the discharge phase. Each of the first powersource 40 and the second power source 90 defines a connected powersource when the respective power sources 40, 90 are connected to theelectrical storage system 80. Each of the first power source 40 and thesecond power source 90 defines a disconnected power source when therespective power sources 40, 90 are disconnected from the electricalstorage system 80. One or both of the first power source 40 and thesecond power source 90 can be connected to or disconnected from theelectrical storage system 80.

In the illustrated construction, the first plurality of batteries 95comprises existing batteries of the vehicle 10, and the second pluralityof batteries 100 comprises separate, additional batteries for thevehicle 10. In other constructions, the first plurality of batteries 95and the second plurality of batteries 100 may be included on the vehicle10 as separate batteries that are provided in addition to existingvehicle batteries.

In order to simplify the description, the various constructionsdescribed herein focus on charging each of the second plurality ofbatteries 100. However, the disclosed circuits and methods could also beused to charge each of the first plurality of batteries 95. In addition,the power for charging each of the second plurality of batteries 100 cancome from a number of sources, including one or more of the firstplurality of batteries 95, the first power source 40, the second powersource 90, or various combinations of these and other power sourcesconnected to the vehicle.

A vehicle electrical system according to various constructions includesone or more of the first plurality of batteries 95, the second pluralityof batteries 100, one or more electrical loads (e.g. lights or HVACsystem 55), and the alternator 35. In addition, the vehicle electricalsystem can also include the second power source 90. As described furtherbelow in regard to FIGS. 2-10, the vehicle electrical system may alsoinclude one or both of a bi-directional battery voltage converter 200and an electronic switch 210 associated with one or more of the first orsecond plurality of batteries 95, 100. In each embodiment, the commonconnections between electrical system components may be referred togenerally as the electrical system, or system “bus.”

FIG. 2 shows a circuit diagram for a portion of a vehicle electricalsystem 150 that includes a construction of the bi-directional batteryvoltage converter 200. The bi-directional battery voltage converter 200includes four switches A, B, C, D arranged as an “H-bridge” coupled byan inductor L. In various constructions, the switches A, B, C, D areelectronically-controlled switches that are capable of carrying largeamounts of current, for example field-effect transistor (FET) switchessuch as metal oxide semiconductor FETs (MOSFETs). Typically, switchesare activated in pairs, for example switches A and D are turned on atthe same time, or switches B and C are turned on at the same time. Whenswitches A and D are turned on, the inductor L is charged from one ormore of the first plurality of batteries 95 (or, alternatively, thefirst power source 40 or the second power source 90 shown in FIG. 1).Switches A and D are subsequently turned off and switches B and C areturned on, allowing the stored energy from the inductor L to bedelivered to one or more of the second plurality of batteries 100.

In some constructions, the switches A, B, C, D are operated by a PulseWidth Modulation (PWM) control 300 (FIG. 2). For example, the outputsOUT-Q and OUT-Q′ of the PWM control 300 may be selectively connected tothe switches A, B, C, D using a routing circuit 310. The routing circuit310 includes two multiplexers 320 a, 320 b which have the OUT-Q andOUT-Q′ lines of the PWM control 300 as inputs as well as a modeselection input from a data interface 330. In the construction shown inFIG. 2, the OUT-Q line is connected to input I1 of multiplexer 320 a andto input I2 of multiplexer 320 b, while the OUT-Q′ line is connected toinput I2 of multiplexer 320 a and to input I1 of multiplexer 320 b. Theoutputs of the multiplexers 320 a, 320 b are then connected to theswitches A, B, C, D.

In the construction shown in FIG. 2, multiplexer 320 a is connected toswitches A and D and multiplexer 320 b is connected to switches B and C.Also in the construction shown in FIG. 2, the routing circuit 310 iscontrolled by the data interface 330, which changes the mode of each ofthe multiplexers 320 a, 320 b so as to route either the S1 inputs or theS2 inputs of the respective multiplexers 320 a, 320 b to the switches A,B, C, D. The data interface 330 includes input/output (I/O) lines, whichare connected to a centralized control system 340, such as shown in FIG.4. The control system 340 may be attached to the vehicle 10 as astandalone unit or as a part of a computer control system for thevehicle 10.

Thus, using the bi-directional battery voltage converter 200 circuitshown in FIG. 2, a single control line (i.e. the “mode” line from thedata interface 330), which puts out a binary signal (e.g. a voltagesignal that toggles between a low value and a high value such as 0 and 1volt), can be used to change the positions of the switches A, B, C, D soas to link the inductor L to either the first plurality of batteries 95or the second plurality of batteries 100. Nevertheless, other methods ofcontrolling the open and closed states of the switches A, B, C, D arealso possible.

Depending on the order of charging and discharging of the inductor L,the first plurality of batteries 95 can be used to charge the secondplurality of batteries 100, or the second plurality of batteries 100 canbe used to charge the first plurality of batteries 95. Further, in placeof the first plurality of batteries 100, power from the first powersource 40 or the second power source 90 may be used to charge theinductor L and thus provide power to the second plurality of batteries.The charging and discharging of the inductor L is typically performed ina cyclic manner, so as to provide an ongoing source of electrical energyto whichever battery or batteries are receiving the energy. The inductorL undergoes repeated cycles of charging and discharging, which invarious constructions occurs at rates of up to 50 kHz or more, with thecapacitors C_(in) and C_(out) helping to build charge and to smooth thevoltage signal. Using the bi-directional battery voltage converter 200shown in FIG. 2, the inductor L can deliver an output voltage to one ormore of the batteries which is lower than, equal to, or greater than thesource voltage connected to the inductor L. The output voltage frominductor L is based on the duty cycles of the switches A, B, C, D, thatis, it is based on how much time per cycle the switches A and D areclosed and connected to inductor L compared to how much time per cyclethe switches B and C are closed and connected to inductor L, as well asthe length of the cycle.

In the constructions shown in FIGS. 2-5, the use of the bi-directionalbattery voltage converter 200 or an electronic switch 210, or both,allows each of the second plurality of batteries 100 to be individuallyconnected or disconnected to the vehicle electrical system 150 forcharging or isolation. The circuit shown in FIG. 3 includes currentsensing (I_(sense)) and voltage sensing (V_(sense)) capabilities. In theconstruction shown in FIG. 3, current sensing is provided by acurrent-sensing resistor R in series with the inductor L. The output ofthe current-sensing resistor R is fed into the PWM control. In otherconstructions, current sensing may be provided by a Hall effect sensor.Voltage sensing (V_(sense)) is provided by a line from the“OUTPUT/INPUT” to the PWM control. The construction of FIG. 3 alsoincludes voltage set (V_(set)) and current set (I_(set)) controls on thedata interface 330 and PWM control 300 to permit setting of particularvoltage and current levels for charging and discharging the particularbattery, e.g. one of the second plurality of batteries 100, that isconnected to the circuit.

FIG. 4 shows a diagram of a vehicle electrical system 150 according to aconstruction of the invention in which the second plurality of batteries100 is coupled to the electrical system of a vehicle using thebi-directional battery voltage converter 200. Although FIG. 4 showsthree batteries 100 a-c, any number may be used. Each of the secondplurality of batteries 100 a-c can be individually charged by thevehicle electrical system 150, for example, by one or both of thevehicle alternator 35 and at least one of the first plurality ofbatteries 95. The various elements of the bi-directional battery voltageconverters 200 shown in FIGS. 2 and 3, including the switches A, B, C,D, the inductor L, the capacitors C_(in) and C_(out), the routingcircuit 310, the PWM control 300, and the data interface 330, may bedistributed between the control system 340 unit and the bi-directionalbattery voltage converter 200 a-200 c units of FIG. 4. One non-limitingexample is that the PWM control 300, the routing circuit 310, and thedata interface 300 may be housed in the same unit as the control system340.

In various constructions, charging is applied to each of the secondplurality of batteries 100 based on the battery's state of discharge.For example, a “bulk” charging stage, which supplies a fixed current torapidly recharge to a partial-charge point, can be used with batteriesthat are relatively depleted, while an “adsorption” stage, in which thevoltage is held constant while supplying varying levels of current, canbe used to complete charging. Finally, a “float” charging stage, whichmeasures the battery voltage and recharges the battery as needed to keepthe battery within a predetermined voltage range, can be used tomaintain battery charge over an extended period of time. Other batterycharging stages are possible and are encompassed within the invention.Using a multiple stage charging method such as that described above isgenerally the most rapid way to recharge a battery while maintainingmaximum battery life.

A drawback of hardwiring multiple batteries in parallel into a singleoperational unit is that all of the batteries are chargedsimultaneously. Under certain conditions (e.g. if one or more of thebatteries has become very depleted or if the load of the vehicleelectrical system is large), recharging of all of the batteries may drawso much power from the vehicle alternator 35 that the alternator 35 isunable to provide sufficient power to the connected electrical loads.

Thus, in one construction, each of the second plurality of batteries 100is separately connected to the vehicle electrical system during therecharging phase in order to prevent too much current from being drawnfrom the alternator 35 through the vehicle electrical system 150. In oneconstruction, each of the second plurality of batteries 100 can becoupled to the vehicle electrical system using an electronic switch 210(FIG. 5). In this and other constructions which reference the use of anelectronic switch 210, however, each of the second plurality ofbatteries 100 can be coupled to the vehicle electrical system 10 usingthe bi-directional battery voltage converter 200 (FIG. 4) instead of anelectronic switch 210. The bi-directional battery voltage converter 200can operate as a switch, for example by not alternating the open andclosed states of the switches A, B, C, D, or by including a mode inwhich all of the switches A, B, C, D are open.

In various constructions, the electronic switches 210 are controlled bythe control system 340, which also monitors the voltage of the vehicleelectrical system 150. Opening a particular electronic switch 210isolates the battery associated therewith. Thus, when the alternator 35is operating (e.g. if the prime mover 25 is in the “On” state) itprovides charging current for the connected load 160, where in someconstructions the load 160 may include one or more of the firstplurality of batteries 95, insofar as these are being charged.

In one construction, the control system 340 initially opens (i.e.disables/disconnects) each electronic switch 210 so as to preventcurrent into or out of each of the respective second plurality ofbatteries 100. The control system 340 then closes (i.e.enables/connects) each switch 210 one at a time, thereby connecting eachof the second plurality of batteries 100 in parallel with the vehicleelectrical system 150. The control system 340 monitors the voltage(V-IN; see FIG. 5) of the vehicle electrical system 150 each time one ofthe electronic switches 210 is closed and an additional battery isconnected. If connection of a particular battery causes the voltage ofthe vehicle electrical system 150 to traverse a threshold then theelectronic switch 210 associated with the particular battery is openedso as to disconnect the battery from the vehicle electrical system 150.In one example, the threshold may be traversed when the voltage dropsbelow a predetermined value (e.g. below 7 volts in a 12 volt system). Anexcessive drop in voltage may indicate that the alternator 35 or otherpower source has been overloaded. In other constructions, a low voltagevalue may be based on a voltage difference from a starting or nominalvalue. In still other constructions, the low voltage value may bedetermined as a percentage or ratio of a starting or nominal value.

Once each of the electronic switches 210 has been tested and left in anopen or closed state, then any remaining switches 210 that were leftopen (e.g. due to the system voltage dropping too low when the switchwas closed) are re-tested. Each of the remaining open switches 210 isclosed one at a time, and the voltage of the vehicle electrical system150 is then measured by the control system 340 to determine whether thevoltage is too low, as discussed above. This procedure is repeated untilall of the switches 210 are closed.

In some constructions, the voltage of the vehicle electrical system 150is continuously monitored and if the voltage drops (e.g. if anadditional electrical load such as the HVAC system 55 is added to thevehicle electrical system 150), then one or more electronic switches 210may be opened until the voltage increases to an acceptable value. Whilethe alternator 35 is operating, the electrical load 160 connected to thevehicle electrical system 150 may change due to factors such as one ormore batteries becoming sufficiently charged so that it draws less powerfrom the alternator 35, or by changes in the use of devices that consumepower, such as lights or the HVAC system 55.

Another possible drawback of hardwiring multiple batteries in parallelinto a single operational unit is that there may be slight imbalances inthe electrical properties within each battery. These imbalances can leadto other problems including a large variance of current delivered byeach battery, reduced battery lifetime, and possible degradation of allconnected batteries due to the presence of one or more defectivebatteries in the vehicle electrical system 150.

Thus, various constructions of the system include methods of balancingthe current that is supplied by each of the second plurality ofbatteries 100 to the vehicle electrical system 150. The method can beimplemented using a system such as that shown in FIG. 4 in which each ofthe second plurality of batteries 100 is connected to the vehicleelectrical system 150 using a bi-directional battery voltage converter200 circuit such as that shown in FIG. 3.

As discussed above, in certain constructions the bi-directional batteryvoltage converter 200 includes a data interface 330 which in turnexchanges commands from the control system 340 (FIGS. 3 and 4). Thecontrol system 340 monitors the voltage (V_(sense)) and current(I_(sense)) conditions of each bi-directional battery voltage converter200. In various constructions, the control system 340 then issuescommands to each bi-directional battery voltage converter 200 to operatewith parameters such that each of the second plurality of batteries 100delivers a proportionate amount of the cumulative total current that issupplied to the connected electrical load 160. In the case where eachbattery has the same nominal characteristics, then appropriate commandsare sent to each bi-directional battery voltage converter 200 such thateach of the second plurality of batteries 100 delivers an equal amountof current to the connected electrical load 160. For example, if theamount of current being delivered by a first battery is greater than thecurrent being supplied by any of the other batteries (e.g. due to thefirst battery having a lower internal resistance or being charged upmore than the other batteries), then the duty cycle of thebi-directional battery voltage converter 200 associated with the firstbattery may be adjusted to reduce the amount of current being deliveredby the first battery.

In certain constructions, the voltage level of each of the secondplurality of batteries 100 is monitored and the bi-directional batteryvoltage converter 200 associated with a given one of the secondplurality of batteries 100 is disabled if the battery is too deeplydischarged, e.g. if the terminal voltage of the battery drops below apredetermined value, for example below 10.5 volts on a battery rated at12 volts. If this were to happen, then the parameters for each of thebi-directional battery voltage converters 200 connected to the remainingfunctional batteries would be adjusted by the control system 340accordingly (e.g. by altering the duty cycle of the bi-directionalbattery voltage converter 200) so as to supply proportionate amounts ofcurrent to the connected electrical load 160.

Still another possible drawback of hardwiring multiple batteries inparallel into a single operational unit is that all of the batteries aredischarged concurrently when the prime mover 25 is turned off and thealternator 35 is no longer providing power to the vehicle electricalsystem 150. Thus, after a period of time supplying the electrical loadrequirements of the vehicle 10, all of the batteries may becomedischarged, possibly leaving no battery power available for criticalloads such as powering the starter motor to re-start the vehicle 10.

Therefore, in various constructions the system includes methods ofmaintaining a minimum charge level in one or more of the secondplurality of batteries 100. In one construction, one or more of thesecond plurality of batteries 100 has an electronic switch 210 connectedin series therewith so that when the switch 210 is opened, the batteryis isolated from the vehicle electrical system 150 (FIG. 5). Asdiscussed above, the bi-directional battery voltage converter 200 canalso serve the role of an electrical switch.

FIGS. 6, 7 and 8 illustrate still other constructions of a vehicleelectrical system 150 according to the invention. In theseconstructions, multiple batteries are associated with a bi-directionalmulti-battery voltage converter 350. In the illustrated constructions ofFIGS. 6, 7, and 8, three batteries 100 are associated with eachbi-directional multi-battery voltage converter 350, though in otherconstructions fewer batteries or more batteries may be associated witheach converter.

In the construction illustrated in FIG. 6, a bi-directionalmulti-battery voltage converter 350 functions in a substantially similarmanner to the bi-directional battery voltage converter 200 illustratedin FIG. 2 and described above. Unlike the construction of FIG. 2,however, an electronic switch 360 is provided between MOSFET switch Cand the associated batteries 100. The electronic switch 360 selectivelyconnects between one or more individual batteries by selectivelyenergizing or de-energizing relays associated with each battery. Therelays are used to switch between multiple separate auxiliary batteries100.

The electronic switch 360 is controlled by an output control of the datainterface 330. The signal of the output control controls the electronicswitch 360 to select which battery 100 is to be charged, discharged orisolated depending on state of charge and other factors.

In the construction shown in FIG. 7, current sensing is provided by acurrent-sensing resistor R in series with the inductor L. The output ofthe current-sensing resistor R is fed into the PWM control. In otherconstructions, a Hall effect sensor may be substituted for thecurrent-sensing resistor R. A voltage sense signal (V_(sense)) isprovided by a line from the “OUTPUT/INPUT” to the PWM control. V_(sense)may also be provided by a line from the “INPUT/OUTPUT” to the PWMcontrol. The construction of FIG. 7 also includes voltage set (V_(set))and current set (I_(set)) controls on the data interface 330 and PWMcontrol 300 to permit setting of particular voltage and current levelsfor charging and discharging the particular battery, e.g. one of thesecond plurality of batteries 100, that is connected to the circuit.

FIG. 8 illustrates an arrangement of bi-directional multi-batteryvoltage converters 350 which is similar to that illustrated in FIG. 4.Instead of having one battery 100 associated with one bi-directionalbattery voltage converter 200, a plurality of batteries 100 isassociated with each bi-directional multi-battery voltage converter 350.

FIGS. 9-10 illustrate still another embodiment of the invention. In thisconstruction, a bi-directional multi-battery voltage converter 350,similar to that of FIGS. 6-8, is incorporated into a “Smart ChargingModule” or SCM 370. The SCM 370 is a construction of a bi-directionalDC-DC converter which can transfer energy between a primary power source(e.g., an alternator) and either of two separate auxiliary batteries 100a, b. In other constructions, the SCM may be configured to switchbetween more than two auxiliary batteries.

As shown in FIG. 9, a micro-controller 380 is operable to selectivelyenergize or de-energize a first relay 390 a and a second relay 390 b. Inother constructions, other electronic switching devices may besubstituted for the relays 390 a and 390 b. In the illustratedconstructions, relay 390 a is associated with battery 100 a, and relay390 b is associated with battery 100 b. When either relay is energized,the associated battery is electrically coupled to the bi-directionalbattery voltage converter by the closing of the relay. Themicro-controller 380 serves a switching function similar to that of theelectronic switch 360 in FIGS. 7 and 8. The micro-controller 380 isconfigured such that either relay may be selectively energized, but bothrelays cannot be energized simultaneously. In some constructions, themicro-controller may also incorporate the pulse-width modulation 300 anddata interface 330 functions of other constructions.

FIG. 10 illustrates a vehicle electrical system 410 incorporatingmultiple SCMs 370. The vehicle electrical system 410 has an ignitionswitch 420, a starter 430, a plurality of vehicle batteries 95, and aconnection for external 120 VAC shore power 440. An inverter 450 andsocket 460 are provided for supplying external 120 VAC loads. Analternator 35 provides power to the electrical system when a prime moveris operating, such as when the vehicle is on the road or idling.

An all-electric HVAC unit 470 is provided, which may be powered by theSCMs 370 when the prime mover 25 is stopped and shore power isunavailable. The HVAC unit 470 incorporates a main controller 480, acompressor assembly 490, and an evaporator fan 510. A human machineinterface 520 provides user input to the HVAC unit 470 to control suchfunctions as temperature and fan speed.

Each SCM 370 and an associated pair of batteries 100 a, b areincorporated into a power management unit 530. In the illustratedvehicle electrical system 410, two power management units 530 areprovided.

The SCM 370 can operate in one of three states. When in a charge mode,the SCM 370 will charge batteries 100 a, b using vehicle batteries 95and alternator 35 as the power source. When in a discharge mode, the SCM370 will deliver power from batteries 100 a, b to the vehicle electricalsystem 410 and associated loads, including the HVAC unit 470. The SCM370 can also be operated in a null mode, where there will be no currentflowing between the auxiliary batteries 100 a, b and the rest of thevehicle electrical system 410.

The SCM 370 mode can be determined by the main controller 480 of theHVAC unit 470. When the vehicle ignition switch 420 is closed, the maincontroller 480 will switch the SCMs 370 to the charging mode. When thevehicle ignition switch is open, the main controller will switch theSCMs to a discharging mode. External inputs, such as a user input to thehuman machine interface 520, can manually select the null, charge, ordischarge modes as well.

Additionally the SCM 370 functions as a three-stage charger while in thecharge mode. The profile of the typical three-stage charger is shown inFIG. 11. In the bulk stage 540, charge current is approximatelyconstant, while charge voltage rises. In the absorption stage 550,charge current decreases while the charge voltage is held at a constant,elevated level. In a float stage 560, both charge voltage and currentare held constant.

FIG. 12 is a flow chart illustrating the battery selection logic of themain controller 480 and SCM 370 in the charge mode. The main controller480 enters the charge mode upon the ignition switch 420 closing. The SCM370 is set up to energize relay 390 a first, thereby connecting battery100 a. In some embodiments, the main controller 480 may incorporate amemory module which tracks usage data for each battery. If the maincontroller 480 determines that battery 100 a received charging priorityover battery 100 b too often, the main controller 480 may override theSCM 370 and select battery 100 b to charge first.

In certain embodiments, the main controller 480 may additionally beprogrammed with a rapid recharge function. In these embodiments, asingle battery 100 a or 100 b of one SCM 370 may be preferentiallycharged by de-energizing all other relays 390 associated with otherbatteries 100. In some embodiments, the main controller 480 mayprioritize charging of a battery 100 with the lowest state of charge(i.e., the battery most in need of charging). Alternatively, the maincontroller 480 may prioritize charging of a battery 100 with the higheststate of charge (i.e., the battery that can be fully charged in theshortest period of time).

After battery 100 a has charged for a period, SCM 370 determines stateof charge. If the battery is fully charged, the SCM will de-energizerelay 390 a and energize relay 390 b, thereby connecting battery 100 bfor charging. Even if the battery is not fully charged, the maincontroller 480 may switch to charging battery 100 b based on othercriteria such as balancing state of charge. The iterative processcontinues until both batteries are fully charged.

FIG. 13 is a flow chart illustrating entry into the null mode. When thenull mode is selected, the main controller 480 signals the SCM tode-energize the relays 390 a and 390 b associated with both batteries100 a and 100 b.

FIG. 14 is a flow chart illustrating the battery selection logic of themain controller 480 and SCM 370 in the discharge mode. The maincontroller 480 enters the discharge mode upon the ignition switch 420opening. The SCM 370 is set up to energize relay 390 a first, therebyconnecting battery 100 a to discharge first. If the main controller 480determines that battery 100 a has a greater discharge history thanbattery 100 b, the main controller 480 may override the SCM 370 andselect battery 100 b to charge first.

After battery 100 a has discharged for a period, SCM 370 determinesstate of charge. If the battery is fully discharged, the SCM willde-energize relay 390 a and energize relay 390 b, thereby connectingbattery 100 b for discharging. Even if battery 100 a is not fullydischarged, the main controller 480 may switch to charging battery 100 bbased on other criteria such as balancing state of charge or maximizingbattery life by preventing deep discharges. The iterative processcontinues until both batteries are fully discharged or the maincontroller returns to the charge or null mode.

The SCM 370 and/or main controller 480 may be programmed with additionalbattery switching criteria, such as a current limit. A current limitpoint protects the auxiliary batteries from an excessive discharge rateand promotes current sharing between the auxiliary batteries.

Accordingly, the invention provides a new and useful control system forelectrical storage elements of a vehicle, which includes a system forcontrolling power into and out of the electrical storage elements.

The invention claimed is:
 1. A method of charging batteries in a vehicleelectrical system, the vehicle electrical system comprising a systembus, a first switch selectively connecting a first battery with thesystem bus, a second switch selectively connecting a second battery withthe system bus, and a controller monitoring a voltage of the vehicleelectrical system and controllably opening and closing the first switchand the second switch, the method comprising: a) opening the firstswitch and the second switch until the first switch and the secondswitch are open at the same time, thereby disconnecting the firstbattery and the second battery from the system bus; b) closing the firstswitch, thereby connecting the first battery with the system bus; c)monitoring the voltage of the vehicle electrical system in transientresponse to the first switch closing; d) opening the first switch if thevoltage of the vehicle electrical system traverses a threshold value intransient response to the first switch closing; e) closing the secondswitch, thereby connecting the second battery with the system bus; f)monitoring the voltage of the vehicle electrical system in transientresponse to the second switch closing; and g) opening the second switchif the voltage of the vehicle electrical system traverses the thresholdvalue in transient response to the second switch closing; and repeatingthe steps of b, c, d, e, f, and g until the first switch and secondswitch remain closed.
 2. The method of claim 1, wherein at least one ofthe first switch and the second switch is an electronic switch.
 3. Themethod of claim 2, wherein the electronic switch includes abi-directional voltage converter.
 4. The method of claim 3, wherein thebi-directional voltage converter includes an inductive H-bridge circuit.5. The method of claim 1, wherein the threshold value is based upon avoltage difference from a starting value.
 6. The method of claim 1,wherein the threshold value is based upon a voltage difference from anominal value.
 7. The method of claim 1, wherein the threshold value isbased on a ratio of the voltage of the vehicle electrical system to astarting voltage.
 8. The method of claim 1, wherein the threshold valueis based on a ratio of the voltage of the vehicle electrical system to anominal value.
 9. A controller for use with a vehicle electrical system,the controller comprising a voltage input terminal, a memory, and amicro-processor, the controller controllably opening a first switch anda second switch until the first switch and the second switch are open atthe same time, thereby disconnecting a first battery and a secondbattery from a system bus, controllably closing the first switch,thereby connecting the first battery with the system bus, receiving avoltage of the vehicle electrical system at the voltage input terminalin transient response to the first switch closing, comparing, in themicro-processor, a value related to the voltage of the vehicleelectrical system to a threshold value from the memory, controllablyopening the first switch in transient response to the voltage of thevehicle electrical system traversing the threshold value, controllablyclosing the second switch, thereby connecting a second battery with thesystem bus, receiving a voltage of the vehicle electrical system at thevoltage input terminal in transient response to the second switchclosing, comparing, in the micro-processor, a value related to thevoltage of the vehicle electrical system to the threshold value, andcontrollably opening the second switch in transient response to thevoltage of the vehicle electrical system traversing the threshold value,wherein the controller continues controllably closing the first switch,thereby connecting the first battery with the system bus, receiving thevoltage of the vehicle electrical system at the voltage input terminalin transient response to the first switch closing, comparing, in themicro-processor, the value related to the voltage of the vehicleelectrical system to the threshold value from the memory, controllablyopening the first switch in transient response to the voltage of thevehicle electrical system traversing the threshold value, controllablyclosing the second switch, thereby connecting the second battery withthe system bus, receiving the voltage of the vehicle electrical systemat the voltage input terminal in transient response to the second switchclosing, comparing, in the micro-processor, the value related to thevoltage of the vehicle electrical system to the threshold value, andcontrollably opening the second switch in transient response to thevoltage of the vehicle electrical system traversing the threshold valueuntil the first switch and the second switch remain closed.
 10. Avehicle electrical system comprising; a system bus; a first battery; afirst switch selectively connecting the first battery to the system bus;a second battery; a second switch selectively connecting the secondbattery to the electrical system; and a controller controllably openingthe first switch and the second switch until the first switch and thesecond switch are both open at the same time, thereby disconnecting thefirst battery and the second battery from the system bus, controllablyclosing the first switch, thereby connecting the first battery with thesystem bus, monitoring a voltage of the vehicle electrical system intransient response to the first switch closing, controllably opening thefirst switch if the voltage of the vehicle electrical system traverses athreshold value in transient response to the first switch closing,controllably closing the second switch, thereby connecting a secondbattery associated with the second switch with the system bus,monitoring the voltage of the vehicle electrical system in transientresponse to the second switch closing, and opening the second switch ifthe voltage of the vehicle electrical system traverses the thresholdvalue in transient response to the second switch closing, wherein thecontroller continues controllably closing the first switch, therebyconnecting the first battery with the system bus, monitoring the voltageof the vehicle electrical system in transient response to the firstswitch closing, controllably opening the first switch if the voltage ofthe vehicle electrical system traverses the threshold value in transientresponse to the first switch closing, controllably closing the secondswitch, thereby connecting the second battery associated with the secondswitch with the system bus, monitoring the voltage of the vehicleelectrical system in transient response to the second switch closing,and opening the second switch if the voltage of the vehicle electricalsystem traverses the threshold value in transient response to the secondswitch closing, until the first switch and second switch remain closed.11. The vehicle electrical system of claim 10, wherein the first switchis a bi-directional voltage converter.
 12. The vehicle electrical systemof claim 11, wherein the bi-directional voltage converter includes aninductive H-bridge circuit.
 13. The method of claim 10, wherein thethreshold value is based upon a voltage difference from a startingvalue.
 14. The method of claim 10, wherein the threshold value is basedupon a voltage difference from a nominal value.
 15. The method of claim10, wherein the threshold value is based on a ratio of the voltage ofthe vehicle electrical system to a starting voltage.
 16. The method ofclaim 10, wherein the threshold value is based on a ratio of the voltageof the vehicle electrical system to a nominal value.